1. Field of the Invention
This invention relates to processes for manufacturing masks for use in x-ray photolithography and more specifically to processes used to manufacture masks which include an x-ray transparent membrane affixed to a support and the resulting structure.
2. Description of the Prior Art
It is known in the art to manufacture integrated circuits using photolithographic processes. In one such process, a wafer of semiconductor material, typically silicon, is covered with a photoresist layer. (Other semiconductor materials, e.g. gallium arsenide, are also used.) The photoresist layer is selectively exposed to light and thereafter developed. If the photoresist is positive resist, the exposed portions of photoresist are then removed, thus exposing portions of the silicon wafer. If the photoresist is negative resist, the unexposed portions of the photoresist are removed, thus exposing portions of the silicon wafer. The exposed portions of silicon wafer are then subjected to further processing to give to the silicon wafer desired electrical properties. It is known in the art to selectively expose portions of the photoresist by using a mask. Typical photolithographic processes use visible light to expose photoresist. It is known, however, that one can obtain finer resolution by using radiation having a wavelength shorter than that of visible light. For example, U.S. Pat. No. 3,743,842 issued to Smith et al. discusses a photolithographic process using radiation in the soft x-ray region of the spectrum.
Masks used in x-ray photolithography typically require an x-ray transparent membrane which supports a patterned layer of an x-ray opaque substance such as gold. A prior art process for manufacturing such a mask is illustrated in FIGS. 1a through 1f. Referring to FIG. 1a, all surfaces of a silicon substrate 10 are coated with a boron nitride layer 12 using a low pressure chemical vapor deposition process (LPCVD). The structure of FIG. 1a is then bonded to a pyrex ring 14 using, for example, an epoxy adhesive as illustrated in FIG. 1b. Referring to FIG. 1c, a circular portion of boron nitride layer 12 on the surface of the silicon adjacent the pyrex ring is removed using any of a number of processes (e.g., a chemical or reactive ion etch) thus exposing a portion of silicon substrate 10.
Referring to FIG. 1d, the portion of silicon substrate 10 exposed by removal of boron nitride is in turn removed, e.g., by a KOH etch. The resulting structure consists of a boron nitride membrane 12 covering a silicon ring 10 which is bonded to a pyrex ring 14. The boron nitride membrane 12 is typically coated with a polyimide layer 16 which provides added mechanical support to boron nitride membrane 12 (FIG. 1e). In addition, subsequently deposited tantalum adheres better to polyimide layer 16 than to boron nitride. Thereafter, a tantalum layer 18 and a gold layer 20 are deposited on polyimide layer 16 and patterned (FIG. 1f). As is known in the art, gold is x-ray opaque and tantalum permits gold layer 20 to adhere to polyimide layer 16. The resulting structure is a mask for use in x-ray photolithography. Although this process provides adequate masks, it is complicated and expensive.